Recently, silver halide emulsions containing tabular silver halide particles having a larger aspect ratio (=ratio of diameter to thickness of the particles) and having excellent spectral sensitivity and sharpness have been developed for use in silver halide photographic emulsions to be used in the photographic field. Silver halide emulsions of this kind have some defects in that, even though they are subjected to an optimum spectral sensitization by using a conventional sensitizing dye (for instance, benzimidazole), normal images cannot be obtained due to severe variations of sensitivity and gradation resulting from latensification after exposure, and that when they are preserved for a long period of time at a high temperature or at a high temperature and a high humidity, a remarkable increment in fog is inevitable, or the sensitivity becomes lowered with the lapse of time.
As is well known in the art, a spectral sensitization technique is applied to a process for the manufacture of photographic materials, in which a certain kind of a cyanine dye is added to a silver halide photographic emulsion so that the photosensitive wavelength region thereof may be expanded to the side of a longer wavelength. In this case, it is well known that the spectral sensitivity (that is, a sensitivity attained by such spectral sensitization) depends upon the chemical structure of the added sensitizing dye and various properties of the silver halide emulsion such as the halogen composition, crystal habit and crystal system of the silver halide, and the silver ion concentration and hydrogen ion concentration of the silver halide emulsion. In addition, the spectral sensitivity further depends upon some other photographic additives incorporated in the emulsion, such as a stabilizer, an antifoggant, a coating auxiliary, a flocculating agent, a color coupler and a hardener.
In addition, the sensitizing dyes to be used herein are required to that they do not have any adverse mutual effects to and from any other co-existing color couplers than the sensitizing dye or any other co-existing photographic additives, and that they may still keep the stable photographic characteristics even when the photographic materials are preserved for a long period of time.
Moreover, the sensitizing dyes must satisfy further requirement that the photographic materials, after photographically processed, are free from any remaining coloration resulting from the used sensitizing dye. In particular, it is especially important that no remaining coloration occurs in a rapid photographic processing to be carried out within a short period of time (in general, several seconds to several ten seconds).
Furthermore, the sensitizing dyes must satisfy the still further requirement that the occurrence of fog resulting from the used sensitizing dye is as little as possible.
It is well known that the addition of a certain kind of a benzimidazolocarbocyanine dye is extremely effective for the purpose of increasing the greensensitivity of silver halides as disclosed in, e.g., U.S. Pat. Nos. 2,912,329 and 2,739,149 and British Patent Nos. 654,690 and 815,172.
However, conventional benzimidazolocarbocyanine dyes are disadvantageous in that the increment of fog is remarkable when the photographic materials coated with a silver halide emulsion containing the dye are kept under the condition of a high temperature or of a high temperature and a high humidity and that the stability of the photographic materials i.e., stability of the emulsion after coating with the lapse of time is poor, resulting in a noticeable decrease in the sensitivity thereof. Improvement or elimination of the abovedescribed defects involved in the use of the known benzmidazolocarbocyanine dyes, including the increment of fog under the condition of a high temperature or of a high temperature and a high humidity and the decrease of the sensitivity with the lapse of time, is one of important subjects in the techniques for manufacturing photographic light-sensitive materials.